The preparation and use of polysuccinimide and polyaspartic acid and its derivatives have been the subject of numerous publications and patents for a long time.
U.S. Pat. No. 4,839,461 (=EP-A 0 256 366) describes the preparation of polyaspartic acid from maleic anhydride, water and ammonia. This involves maleic anhydride being reacted in an aqueous medium, with the addition of concentrated ammonia solution, and then polymerized. During this polymerization, the mass becomes first highly viscous and then solid-porous, which requires handling which is expensive in terms of process technology.
U.S. Pat. No. 5,296,578 describes the preparation of PSI from maleic anhydride, water and ammonia. Maleic anhydride is hydrolysed in water to give maleic acid and this is then converted into the ammonium salt with concentrated ammonia solution. The water is evaporated off from the solution in a stirred reactor and the monoammonium salt is then polymerized in bulk at temperatures above 170.degree. C. to give PSI. In this process, the mass is converted into solid PSI in the course of several hours via highly viscous phase states, and this is then hydrolysed to give PAA.
U.S. Pat. No. 5,288,783 describes the preparation of PAA from maleic acid or fumaric acid, water and ammonia. This involves maleic anhydride being mixed with water in a stirred tank and converted into maleic acid, while cooling. The maleic acid monoammonium salt is prepared by addition of concentrated ammonia solution. The water contained in the mixture is then evaporated off and the dry monoammonium salt is polymerized at temperatures from 190 to 350.degree. C. An alternative proposal is to process further the monoammonium salt present in aqueous solution to PSI by extrusion at temperatures from 160 to 200.degree. C. The PSI prepared by one of the two process routes is then hydrolysed to PAA under alkaline conditions.
EP-A 593 187 describes the preparation of PSI by thermal polymerization of maleamic acid at temperatures from 160 to 330.degree. C. over a reaction time of 2 minutes to 6 hours. Reference is also made to polycondensation in a solvent using condensation auxiliaries.
DE-A 4 023 463 (U.S. Pat. No. 5,142,062) describes a process for the preparation of poly-succinimide in the presence of phosphoric acids. DE-A 4 221 875 (=WO 94/01486) describes the preparation of so-called modified polyaspartic acids by poly-condensation of aspartic acid with other compounds, if appropriate in the presence of phosphoric acids or derivatives thereof. It is known from WO 95/02007 to prepare polymers of aspartic acid by heating maleic anhydride and acid derivatives of ammonia. Phosphoric acid, inter alia, is mentioned as the acid. Of the corresponding salts, however, only diammonium hydrogen phosphate and monoammonium dihydrogen phosphate are mentioned. EP-A 604 813 (DE-A 4 244 031) discloses a process for the preparation of polysuccinimide and polyaspartic acid from maleamic acid in which maleic anhydride and ammonia are reacted to give maleamic acid and the maleamic acid is subjected to polymerization, which can be carried out in the presence of phosphoric acids and polyphosphoric salts.
WO 96/34908 describes a process for preparing polymers having repeating succinyl units, especially polysuccinimide and polyaspartic acid, using triammonium salts, especially triammonium phosphates. According to Example 1, at 180.degree. C. and with a reaction time of 2 h 45 min, 95 g of polysuccinimide (PSI) having a molecular weight of 2580 are obtained; in Example 2, at 240.degree. C. and with a reaction time of 2 h 50 min, 99 g of PSI of MW 2550 are obtained; in Example 3, at 240.degree. C. and in 4 h 40 min, 88 g of PSI of MW 2096 are obtained, and in Example 4 at temperatures of 200.degree. C. over 5 h, 104 g of PSI of MW 5858 are obtained. In Example 5, under the same conditions as in Example 4 but with the addition of 10 g of diammonium phosphate, PSI of MW 4566 is obtained.
DE 4 322 410, moreover, shows the route to the synthesis of PSI and PAS via the reaction of maleic anhydride with ammonium carbonate, with heating.
A disadvantage of the known processes is that depending on reaction conditions, such as reaction time and temperature, they lead to very different PSI with varying average molecular weights and, consequently, to products of unsatisfactory degradability.
The present invention is therefore based on the object of providing an improved process for the preparation of polymers with repeating succinyl units, in particular of polysuccinimide and polyaspartic acid, having increased M.sub.W values at non-elevated, moderate synthesis temperatures and at the same time without increased polycondensation times.